This invention relates to manufacturing methods for producing a uniform coating of dielectric material on a short probe or wire, and for use of a short probe so coated for assembling a coaxial-to-waveguide electromagnetic transition.
High power phase shifters may be made by selecting the magnetic characteristics of magnetic materials such as garnets or ferrites located in the transmission path of the electromagnetic propagation. For radar antenna use, many such phase shifters may be configured together as an array. One such arrangement includes a rectangular waveguide including broad conductive walls spaced apart by narrow conductive walls. An elongated hollow garnet (ferrite core) having a rectangular cross-section is dimensioned to fit within the waveguide and to extend from one broad wall of the waveguide to the opposing broad wall. A "latch"wire extends through the hollow center of the ferrite core and is connected for receiving a control signal in the form of direct current pulses, all in known manner.
In the context of an array antenna, each waveguide phase-shifter may be associated with an antenna element. It may be inconvenient to distribute the radio-frequency signals to (and from) each of the phase shifters by means of a waveguide distribution network, because waveguide is bulky, difficult to fabricate and not readily amenable to change, rerouting or repair. Consequently, the signals may be coupled to and from the waveguide phase shifters by means of coaxial cables. Each phase-shifter, therefore, requires a transition from a coaxial transmission line to waveguide (coax-to-waveguide transition). Those skilled in the art know that antennas and transmission lines are reciprocal, and operate in the same manner for flow of signals in either direction. While a transition may be termed a "coax-to-waveguide transition", it is just as much a waveguide-to-coax transition.
Coax-to-waveguide transitions using electric probes are notorious in the art. A coax-to-waveguide transition involves coupling the outer conductor of the coaxial transmission line to the edges of an aperture in a broad wall of the waveguide, and extending the inner conductor of the coax into the waveguide to form what amounts to a small monopole antenna within the waveguide. FIG. 1 is a simplified exploded view of a coax-to-waveguide transition in the context of a phase shifter. In FIG. 1, a rectangular waveguide 10 including broad upper and lower walls 10a and 10b, respectively, and narrow side walls 10c and 10d, is dimensioned to accept an elongated garnet ferrite core illustrated as 12. Ferrite core 12 has a rectangular cross-section, the larger dimension of which is substantially equal to the dimension between broad walls 10a and 10b of waveguide 10. As illustrated in FIG. 1, ferrite core 12 is hollow, defining an elongated rectangular aperture 14 extending through its length. A pair of wires, illustrated together as 16, extends through a ceramic insulator 18 dimensioned to fit within rectangular aperture 14 in ferrite core 12. Wires 16 are adapted for carrying currents in mutually opposite directions for polarizing the core for producing the desired phase shift, all in known fashion.
As also illustrated in FIG. 1, ferrite core 14 defines a narrow slot 20 which extends from upper broad wall 10a to lower broad wall 10b. Slot 20 has a rectangular cross-section, as more clearly shown in FIG. 4b. An aperture 22 formed in upper broad wall 10a of waveguide 10 is located at a position which is centered at the end of slot 20 when ferrite core 12 is mounted within waveguide 10. A coaxial probe assembly designated generally as 24, which is similar to a coaxial panel connector, includes a threaded coaxial SMA connector 26 coupled through a coaxial right-angle transition 28 and coaxial section 30 to a baseplate 32. The center conductor of the coaxial section (not illustrated in FIG. 1) extends beyond baseplate 32 to form probe or "stinger" 34. Baseplate 32 includes mounting holes, one of which is designated 38, which mate with studs or rivets, one of which is designated 40, which are affixed to and extend above upper broad waveguide wall 10a. Studs 40 are spaced relative to aperture 22 so that baseplate 32 of assembly 24 positions probe 34 in slot 20 of core 12 when baseplate 32 is indexed to the studs and bears against the upper surface of waveguide wall 10a.
A waveguide short-circuit terminates the rear of the waveguide and is illustrated in FIG. 1 as a conductive block 46, which is dimensioned to fit within the near end of waveguide 10. Holes illustrated as 48 are indexed with matching clearance holes, one of which is illustrated as 50, so that short circuit block 46 may be fastened securely in place by means of rivets (not illustrated).
FIG. 2 illustrates waveguide 10 with ferrite core 12 and other parts mounted within, and also illustrates assembly 24 mounted thereon, in phantom for clarity. Elements of FIG. 2 corresponding to those of FIG. 1 are designated by the same reference numerals.
As so far described, signals flowing between waveguide 10 and coaxial connector 26 make the transition by means of probe 34 which extends into slot 20 of ferrite core 12. At high power levels, arcing may take place between probe 34 and adjacent structures, notably ferrite core 12 and the periphery of aperture 22. It is theorized that the arcing is attributable to the relatively low breakdown voltage of air and the small clearance between the probe and its adjacent structures.
In order to provide a higher breakdown voltage between the probe and adjacent structures, an assembly method was used by which a room temperature vulcanizing silicone elastomer (RTV elastomer) was injected by hand into slot 20 just before connector assembly 24 was assembled to the waveguide and ferrite core 12. While this improved the power handling capability of the assembly, unit-to-unit variations in the power handling capability were noted. The unit-to-unit variations in power handling capability were attributed to manual assembly of each connector assembly 24 to its waveguide-ferrite core assembly. In order to rigidly control the assembly procedure, a robotic system was devised by which robotic injection of RTV into slot 20 was followed by robotic insertion of connector assembly 24 to the waveguide-ferrite core assembly. The connector assembly was inserted by translating the connector assembly in a direction parallel with the axis of the probe directly into final mounting position.
Piece-to-piece variations in the performance were still found. When the joints were dissected, voids were found in the cured elastomer. It is believed that a void-free dielectric filling is necessary for consistent high power performance. An improved assembly method is desired.